Update multi package update instructions
    
    Package Update Guide contained strange instructions
    recommending using side tags only for Rawhide and early branched,
    and buildroot overrides for late branched and stable.
    Actually, side tags should be preferred in every case.
    Merged the instructions together to new section "Multi Package Updates"
    that prefers side tags in all cases.
    
    The side tag instructions are also improved slightly:
    
    * Add section about removing side tags.
    * Add a simpler syntax for requesting a side tag for a specified Fedora
      release
    
    Resolves #33